Invacare MK6i User Manual page 66

Turn the levers (1) of both holders until the cams (2) can be pulled out of
the slots on the plates.
Unhook the upper backrest support and reposition it in the desired slots.
Bring the lower backrest support into the desired position.
Turn the levers (1) of both holders until the cams (2) can be slid into the
slots on the plates.
Slide the holder in and turn until it reaches the securing belts.
Secure the lever using the securing belts.
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Lower backrest holder
Retainer plate
Upper backrest support
Lower backrest holder